Currently electrical generators on large civil aircraft are driven by the accessory (external) gearbox. Also on such aircraft cabin pressurisation is generally provided by air bled from the engines. For future aircraft, and particularly those having composite wings, it is not desirable to have hot bleed pipes passing through the wings. Especially with composite wings there would be a potential hazard in the event of a pipe failure.
As an alternative to the above outlined arrangement, cabin pressurisation can be achieved by having compressors onboard the aircraft and powering them by electricity generated by the engine. This would however increase the size of the generators on the engine. Such large generators could still be powered by the engine except that at descent conditions the electrical load will tend to slow the shaft and thus stall the compressor. The descent idle speed of the engine could be increased to prevent such a stall, but this also increases the engine thrust during descent which is undesirable.
Airports generally have the capability to start aircraft engines pneumatically. Usually a ground support cart is provided which has a large compressor or a compressed air tank on board. Not all airports though have the capability to start aircraft engines electrically, and particularly in view of the significant electrical power which may be required for this. Not all aircraft engines can though be started pneumatically, and therefore the use of such aircraft has to be limited to locations where electrical starting is possible.